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A bulb socket is a fixture into which a light bulb is inserted and which transmits electric current to the bulb. A bulb socket is connected to some type of electric current, which can come from any number of sources, such as household current or battery power. Most people are familiar with the standard incandescent light bulb socket, and this is one of the most common types. Hundreds of different light bulb socket designs exist and are used for many different types of light bulb as well.
In 2011, light bulbs are by far the most common electrical device in use in the world, and there are billions of them being used at any given time. Incandescent bulbs are the most common type of light bulb, although high-efficiency fluorescent bulbs are becoming more and more popular. Other types of bulbs, each with their own type of socket, include halogen bulbs and light emitting diode (LED) bulbs.
Most standard incandescent bulbs use a socket consisting of a cylindrical piece of insulating material fitted with an inset metal collar with broad threads. A light bulb has a similar metal collar at its base designed to screw into the socket. A second piece of metal on the bottom of the bulb matches up with a similar contact at the center of the socket cavity. The metal collars and the center-mounted contacts are separated by an insulator, and when the bulb is screwed in to the socket, an electric circuit is completed, causing the element of the bulb to become hot and glow, producing light.
This standard screw-in configuration for most incandescent bulbs is produced in various sizes, with little difference between them except size. Bulb sockets may have ratings, indicating the wattage of the bulbs that may be safely used with them. Smaller sockets of this type are often used in decorative lighting, but are also used in many other applications, such as some flashlights.
Since there is no industry or world-wide standard for bulb design and their corresponding sockets, a wide array of specialized bulb and socket designs have been developed over the last century or so. Bulbs may screw into their sockets with threads, snap in, or be pushed in and secured by clasps or other means. Some sockets have two or more small holes that are made to accept metal prongs on the base of the bulb. No matter what the design of a particular bulb and socket, they are all designed to complete a circuit which integrates and passes current through the bulb, producing light.